Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015–2016 El Niño Event in the Amazon Forest

Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (V(s)), stomatal conductance (g(s)) and leaf water potential (Ψ(L)) of multiple canopy trees, and their correlations with leaf temperature (T(leaf)) and environmental conditions [direct solar radiation, air temperature (T(air)) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of g(s) (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in T(leaf). In the morning, T(leaf) and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as T(leaf) approached a daily peak, allowing Ψ(L) to begin recovery, while sap velocity declined with an exponential relationship with T(leaf). In Manaus, hysteresis indices of the variables T(leaf)-T(air) and Ψ(L)-T(leaf) were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, T(leaf) approached 40°C for all studied species and the differences between T(leaf) and T(air) reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, T(leaf) was higher than T(air) during the middle morning to early afternoon, and lower than T(air) during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in Ψ(L) during the afternoon period giving an observed counterclockwise hysteresis pattern between Ψ(L) and T(leaf).